The present disclosure relates to industrial process control or monitoring systems. More specifically, the present disclosure relates to process variable transmitters of the type which use averaging pitot tube (APT) probes to measure a process variable of an industrial process.
In industrial settings, control systems are used to monitor and control inventories of industrial and chemical processes, and the like. Typically, the control system that performs these functions uses field devices distributed at key locations in the industrial process and coupled to control circuitry in the control room by a process control loop. The term “field device” refers to any device that performs a function in a distributed control or process monitoring system, including all devices used in the measurement, control and monitoring of industrial processes.
Some field devices include a transducer which couples to the process fluid. A transducer is understood to mean either a device that generates an output signal based on a physical input or that generates a physical output based on an input signal. Typically, a transducer transforms an input into an output having a different form. Types of transducers include various analytical equipment, pressure sensors, thermistors, actuators, solenoids, indicator lights, and others.
Field devices, such as process variable sensors used in industrial processes can be installed in the field on pipelines, tanks and other industrial process equipment. Such devices sense process variables such as process fluid flow, process fluid temperature, process fluid pressure, process fluid conductivity, process fluid pH and other process variables. Other types of industrial process field devices include valves, actuators, field controllers, data displays and communication equipment such as industrial field network bridges.
One type of process variable sensor is a flow meter that can measure a rate of fluid flow, for example. One type of flow meter, which employs an averaging pitot tube (APT) primary element, is a popular device for flow measurement because of its ability to be inserted into and retracted from a flow line, its low pressure loss, and reliable performance. The APT primary element senses and averages pressures from multiple locations across a pipeline through which a process fluid is traveling. This average pressure is then used in conjunction with flow theory and experimentally determined quantities to provide a flow measurement for the fluid. One type of APT primary element is the Annubar® APT available from Emerson Process Management. For at least Annubar® type APT primary elements, it is required that the APT primary element span the process pipeline such that multiple samples across a section of the pipeline can be averaged to account for variations in flow across the section.
Because of different customer needs, installation requirements, etc., manufacturers of APT primary elements often have to customize the APT primary elements for their customers. The custom nature of some APT primary element manufacturing results in increased costs. Many APT primary elements are built to order using customer supplied pipe internal diameter and wall thickness dimensions. These dimensions are used to determine the optimal hole and slot pattern on the upstream and downstream faces of the APT primary element. Custom manufacturing provides products which deliver high accuracy measurement but also extends the lead time for APT primary elements on both the order entry and delivery ends of the supply chain.
For some customized APT primary elements, product manufacturing only begins once unit-specific information is received and communicated through the value chain. Customers may struggle to obtain the internal pipe wall thickness and diameter dimensions needed for primary element manufacturing when specifying a flow measurement device because they do not cut a hole in the pipe until it is time to install the flow measurement device. This leads to the common practice of the customer supplying nominal pipe size and dimensions at the time of order entry. Nominal dimensions are typically defined in industry standards, for example in the ASME B36 standards, and possess a degree of uncertainty. Since the customers cannot provide measured values, additional uncertainty, up to several percent in small line sizes, can be introduced to the flow measurement.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.